This invention relates to an error-control system for data transmission and more particularly to an automatic-repeat-request (abbreviated as ARQ) system for controlling the occurrence of errors in a transmission channel when a data block is transmitted.
Error control used with a data transmission system is already known. Errors and error control are set forth in a publication entitled "Errors and Error Control," by H. O. Burton and D. D. Sullivan, Proceedings of the LEEE, Vol. 60. No. 11, November 1972. The automatic-repeat-request (ARQ) system is detailed in a publication entitled "ARQ Error Control on the Satellite Channel," by Allen G. Gatfield, et al, COMSAT Laboratories (International Conference on Communications 1974).
A generally accepted type of the ARQ system divides a series of data into blocks each having a prescribed number of bits, attaches, if necessary, a check code for detection or correction of errors after each data block and transmits the data block and check code in the form of a block signal. With one type of the ARQ system, that is, a continuous ARQ system, a series of data blocks are continuously sent forth from the transmitting side to the receiving side through a transmission channel. When notified of a data transmission error by the receiving side, the transmitting side again delivers to the receiving side a series of data blocks formed of an original data block corresponding to an erroneous data block and other succeeding properly transmitted data blocks. This continuous ARQ system is favorably accepted as economical and efficient.
FIG. 1 illustrates a sequence in which data are transmitted and received in the typical continuous ARQ system. With this system, the receiving side does not make any response (no response) to the transmitting side when supplied with error-free data and issues a negative acknowledgment (NAK) signal only when receiving erroneous data. The transmitting section of the transmitting side supplies the receiving side through a forward channel with a data block formed of a prescribed number of bits and error-detecting bits attached thereto as shown in FIG. 1(a). Now, let it be assumed that a data block 2 is rendered erroneous by a noise occurring in a transmission channel. Then, the receiving section of the receiving side detects the erroneous data block 2 as shown in FIG. 1(b). At this time, the transmitting section of the receiving side supplies the transmitting side through a backward channel with an NAK2 signal as shown in FIG. 1(c) denoting that the data block 2 has not been received in a proper state. The NAK2 signal received by the receiving section of the transmitting side is conducted to the transmitting section of the transmitting side as shown in FIG. 1(d). When supplied with the NAK2 signal, the transmitting section of the transmitting side stops transmission of data blocks as shown in FIG. 1(a), and again delivers to the receiving side a series of data blocks, starting with an original data block corresponding to the erroneous data block 2 with a retransmission signal RIS attached. The receiving side temporarily stops operation, until the data block 2 is received in a corrected form, and discards data blocks previously following the erroneous data block 2.
In the case where the transmitting side receives a signal which is not an NAK signal, the following two processes are customarily applied:
(A) The transmitting side does not make enquiry to the receiving side. Accordingly, the receiving side generally retransmits a second NAK signal, unless the transmitting side supplies the receiving side with a correct group of data blocks in a prescribed length of time after the first NAK signal was sent forth to the transmitting side.
(B) HDLC-High Level Data Link Control System
When supplied with a different signal from the NAK signal, the transmitting side sends forth an enquiring (ENQ) signal to the receiving side.
The above-mentioned steps prevent the stoppage of data transmission between the transmitting and receiving sides, even when the NAK signal issued from the receiving side to the transmitting side is rendered erroneous due to the occurrence of a channel noise on the transmission channel and the NAK is not detected by the transmitting side in a proper form.
However, it sometimes happens that a dummy NAK signal arises by a channel noise appearing on a transmission channel, the said dummy NAK signal is detected by the transmitting side, though the receiving side has not issued any NAK signal. In such case, the receiving side does not under the above-mentioned process (A) deliver an NAK signal to the transmitting side, unless an erroneous data block is detected, but continuously receives data blocks. Upon receipt of a dummy NAK signal, the transmitting side stops transmission of data, until a proper NAK signal is received from the receiving side. This event results in the suppression of data transmission between the transmitting and receiving sides.
the above-mentioned process (B) indeed eliminates the stoppage of data transmission between the transmitting and receiving sides, but has the following disadvantages. In the case where the two-wire circuit is utilized in a full-duplex transmission channel, generally, the transmission side sends forth bit signals to the receiving side through a forward channel at a higher data rate than when the receiving side delivers bit signals to the transmitting side through a backward channel. Therefore, the ENQ signal issued from the transmitting side is very likely to be rendered erroneous. Accordingly, an extra time is required to correct the ENQ signal, so that it will consume a considerable length of time to restore data transmission to a normal state.
As mentioned above, the prior art error-controlling system sends forth a control signal in both forward and backward directions. Should these control signals be rendered erroneous during transit, much time is required to effect transmission of a proper control signal in an error-free state between the transmitting and receiving sides.
Moreover, particular codes concerning the ENQ signal and NOT signal have to be applied.
With the CCITT-V41 system, data should be carried through a transmission loop in a shorter time than when a single data block is transmitted from the transmitting side to the receiving side. With this CCITT-V41 system, the final bit for detecting an error state is reversed as errors sometimes occur due to the occurrence of noises on a transmission channel and it therefore should be ascertained whether an NAK signal is received by the transmitting side in an error-free state. This system does not clearly provide any countermeasure for reception of a dummy NAK signal NAK.